"Community First Responder volunteers are trained to attend emergency calls received by the ambulance service and provide care until the ambulance arrives."
It's essentially someone trained up to advanced first aid and some basic paramedic training, who attends emergency calls where an ambulance may not be able to get to them right away, and can provide care until an ambulance arrives. I suppose it's similar-ish to being an auxiliary fireman, special constable or in the TA
For example, of a night time in Barrow where I live only 1 ambulance is on duty. My brother in law has a phone which is with him constantly. Everytime a call comes through to 999, ambulance control will text him and any other first responders in the area to see if they can go. He's got a defib, oxygen therapy kit etc in his car and can provide basic first aid, assess the situation and direct further ambulance responses.
Saved some old womans life the other day who'd got into cardiac arrest, where the ambulance was tied up elsewhere and took 15minutes to arrive.